1. Field of the Invention
The present invention relates to a data transmission equipment which connects a plurality of data stations via a common transmission line, and more specifically to a data transmission equipment which transmits the data of a process control apparatus cyclically.
2. Description of the Prior Art
The data transmission equipment used for the process control apparatus must be capable of efficiently transmitting at high speeds the data of even small amounts that are sporadically produced by many sensors arrayed on a process line, as well as the data of large amounts produced by a process line control apparatus or the like.
According to the conventional data transmission terminal of this type, a main station 1 is connected to local stations 4a to 4c in the form of a loop through transmission lines 2, as shown in FIG. 1. The main station 1 transmits the data to, or receives the data from, input/output devices 5 that are connected to the local stations 4a to 4c.
FIG. 2 illustrates the internal construction of the local station 4a. Namely, a signal converter 6 converts into parallel signals serial signals transmitted to the local station 4a via transmission line 2. The signal converter 6 further converts parallel signals in the local station 4a into serial signals and transmits them to the transmission line 2. A timing controller 7 takes timing in the local station 4a, and an address memory 8 stores address data from the transmission line 2. A multiplexer 9 selects input data 61 depending upon the address, and a demultiplexer 10 distributes the data transmitted through the transmission line 2 to an output memory 60 depending upon the address. The output memory 60 produces process outputs 62.
Operation of the conventional apparatus of FIGS. 1 and 2 will be described below with reference to FIG. 3.
The main station 1 transmits signals to, and receives signals from, the process input/output devices 5 via local stations 4a to 4c that are connected in the form of a loop through the transmission line 2. As shown in FIG. 3, the main station 1 produces the data, i.e., transmits synchronizing data SYNC, address data ADRS, and data 1 to n corresponding to the address, to the local station 4a through the transmission line 2. After having taken the synchronism, the local stations send the data to the process input/output devices 5 which correspond to each of the addresses or the data are written on slot DATA 1 to n. The main station 1 reads the data that are introduced. The next address data are then transmitted and received. The above-mentioned operation is repeated to transmit and receive all data.
Setup of the local station 4a of FIG. 2 will be mentioned below in further detail. Serial signals introduced through the transmission line 2 are converted into parallel signals by the signal converter 6. In this case, the signals are synchronized relying upon a synchronizing signal that is transmitted for the first time as shown in FIG. 3. The next address signal ADRS is stored in the address memory 8. The data DATA 1 to n transmitted subsequently are input to the output memory 60. The data are written on the output memory 60 specified by addresses, relying upon the demultiplexer 10 and relying upon the signals that are stored in the address memory 8 and that are selected by addresses. The timing controller 7 controls the timing, and the data written on the output memory 60 are transmitted to the process outputs 62. The next synchronizing signal SYNC is transmitted to the transmission line 2, followed by the address signals and the data signals. As a matter of course, the address in this case is +n relative to the address of the previous time, and the data corresponding to the new address are transmitted. The above-mentioned operation is repeated so that all of the output data are transmitted to the local stations 4a to 4c.
Below is mentioned input data 61 transmitted from the process input/output devices 5. The synchronism is maintained by the synchronizing signal transmitted via the transmission line 2, and the address data ADRS are stored in the address memory 8. Depending upon the address ADRS, the multiplexer 9 selects the input data 61 from the process input/output devices, and transmits them to the signal converter 6 which converts parallel signals into serial signals and superposes them on the slots of DATA 1 to n of FIG. 3. The main station 1 reads the data and advances the address by +n, and then reads the next data. The above-mentioned operation is repeated to read all of the input data. That is, the above-mentioned operation is carried out cyclically to transmit all of the input/output data.
According to the above-mentioned conventional apparatus, however, when the outputs are to be produced to many process input/output devices, the data must be written on the output memory 60 through demultiplexer 10 consuming extended periods of time. In worst cases, therefore, the next data will be produced before the data are all written on the output memory 60. When the data are to be introduced, furthermore, the multiplexer is operated after the address is discriminated and, then, the input data is introduced, giving rise to the occurrence of time lag. Consequently, the data may not often be superposed on the specified slots. This becomes a problem particularly when there are many inputs and outputs, and when the addresses and data are to be transmitted through a bus over a long distance.
There has further been proposed a data transmission equipment as shown in FIG. 4. Namely, the data transmission equipment 11 transmits a variety of data related to the process control apparatus 12. The data transmission terminal equipment has data stations 14 to 17 connected through a loop transmission line 13, a display setting board 18 which is connected to the data station 14 and which sets and displays the data, and process line controllers 19 to 21 connected to the data stations 15 to 17. The process line controllers 19 to 21 transmit the data related to process control to, or receive them from, a process control apparatus 12. The data transmission equipment 11 further possesses sensor bases 24 to 26 that are connected to the display setting board 18 via cables 22. To transmit the data through the loop transmission line 13, the data station 14 also functions as a synchronizing station which forms transmission signals consisting of a frame which is shown in FIG. 5.
Each frame has a synchronizing slot SYN and data slots SLT.sub.1 to SLTn that are suitably allocated to the data stations 14 to 17 and that work to transmit the data DATA. The frames are continuously transmitted to the loop transmission line 13.
With the above-mentioned conventional data transmission equipment, however, the sensor bases are connected to the display setting board, resulting in complicated construction. If the sensor bases are connected to the loop transmission line, the data of sensor bases exhibit quite different natures and, hence, the transmission efficiency of the data transmission terminal equipment greatly decreases. Further, the same input signal is introduced by each of the process line controllers, or the signal introduced by any process line controller must be transmitted to other process line controllers through the loop transmission line. Accordingly, the transmission efficiency decreases greatly.